字幕表 動画を再生する 英語字幕をプリント NARRATOR: Humans civilizations have been wonderfully diverse with an amazing variety of unique customs, religious practices, social structures and technologies. These differences are fascinating, but Big History teaches us to look at the past differently, to seek out commonalities and identify patterns that unite seemingly unrelated phenomena. Often this produces insights that are even more interesting. One of the most important patterns we see among agricultural civilizations is the need to expand. Many agrarian civilizations grew to control a lot of territory. The Roman Empire at its peak controlled around 2.5 million square miles, the Persians more than three million square miles and the Mongols a whopping 10 million square miles with an empire stretching across Asia. Though these empires existed in different places and at different times, they shared a strong drive to expand geographically. And when they could no longer grow, their territory shrank and eventually their civilizations collapsed. So why was geographic expansion so essential to their survival? Maintaining a state was expensive. Increasing the large populations required more infrastructure, more resources as well as bigger governments and militaries. Ambitious monumental architecture like the Pyramids added even more expense because land productivity had its limits and leaders could face rebellion if they try to squeeze subjects too much with heavy taxation. There were limits to how much states could grow using internal resources. Thus growth necessitated taking what others had produced rather than trying to increase productivity within the state. During the era of agrarian civilizations this type of military expansion was common. We see it in civilizations around the world with the Persians, the Romans, the Chinese dynasties and with the Aztec and Inca empires in the Americas. Large professional armies required food, weapons and complex infrastructures like roads, forts and defensive walls. All of these came at great expense, which itself increased the need for expansion. Some military innovations lead to inventions that would advance entire civilizations. Iron is a good example. The hard metal was initially used for weapons but once iron plows appeared, crop yields increased, and so did populations. Roads, initially designed to move armies, became important trade routes and building techniques were refined after the construction of so many walls, forts and watch towers. Throughout this era, borders were constantly contested but there was another side to the interactions that took place. Outposts and border regions often became centers for commercial and technological innovation. This was because different cultural groups connected and many non-military exchanges occurred, intensifying collective learning. We see this accelerated change very clearly in Europe when the Roman Empire splintered into a series of small competitive states. This increase in commerce and the exchange of ideas drove a transition of power from large agrarian civilizations to these smaller commercial states. The pace of collective learning further intensified. Traditional agrarian civilizations in other parts of the world lost power and in many cases were colonized by the same European states with dramatic implications for today's world. Empires were shaped less by physical borders and more by spheres of commercial influence. The modern world had begun to take shape.